1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image on a recording medium, and more particularly to an image recording apparatus for achieving said image recording by transferring ink from an ink sheet bearing ink thereon to the recording medium.
Such image recording apparatus includes printers, electronic typewriters, copying machines, word processors, facsimile machines or the like.
2. Related Background Art
Thermal recording apparatus employed in printers, facsimile machines or copying machines generate dot patterns on a recording medium of sheet form such as paper or a plastic sheet, by selectively generating heat in plural dot forming elements provided in a thermal head, according to image data to be recorded. This category of recording apparatus can be divided into a serial printer type in which recording is made by the movement of the recording head in lateral direction of the recording sheet, a line printer type in which recording of one line is made at the same time, or a page printer type in which recording of one page is made at the same time. Also such thermal recording apparatus can be divided into a thermal transfer type in which ink is transferred from an ink ribbon onto a plain paper, and a thermal type in which color is generated by direct heating of a heat sensitive paper with a thermal head.
Such a thermal transfer recording apparatus employs an ink sheet composed of thermal transfer ink containing a coloring material such as carbon black. The ink is coated on a substrate sheet for example of polyethylene terephthalate (PET) and the binder of said thermal transfer ink has been principally composed of waxes. In such case, as the fusion viscosity of the thermal transfer ink is lowered, the fused ink and the surface of the recording sheet have to be in mutual contact at the ink transfer when heat is applied. However, a recording sheet with low surface smoothness reduces the probability or area of contact thereof with the fused ink, so that the ink cannot reach the recesses on the surface of the recording sheet. This phenomenon has resulted often in print break or incomplete edge sharpness.
Therefore, for the purpose of obtaining a satisfactory transferred image even on a recording sheet with low surface smoothness as referred to "rough paper" hereinafter, it has been proposed to employ thermal transfer ink principally composed of a resin with relatively high fusion viscosity, for example ethylene-vinyl acetate copolymer or polyamide and to transfer the ink in a semi-fused film state onto the rough paper. In such method, as the thermal transfer ink has to be peeled off from the substrate sheet while in a state of high coagulating force in order to retain the film state, it is preferable to employ a thermal head having heat generating elements close to the end of the head substrate board, thereby reducing the time from heat application to the peeling of the ink sheet and the recording sheet, as referred to "peeling" hereinafter.
The above-explained process enables image recording of high quality on a rough sheet, but, in the case of recording on a smooth sheet, the ink sheet has to be given a larger tension than in the case of rough sheet, in order to attain complete peeling of the ink sheet and the recording sheet. A tension same as in the case of rough sheet may result in defective ink transfer due to incomplete peeling. In order to solve this problem, it has been thought to apply a high tension at the ink transfer, thereby achieving complete peeling from rough to smooth paper, but, in such case, a stronger mechanism is required because the ink sheet is always under a high tension.